Recently, vacuum apparatus for a fusion reactor, accelerator, or the like, have grown continuously larger in size and have become quite complicated in structure. Welding techniques and sealing techniques by means of a metal gasket, or the like, in the manufacture of a vacuum vessel have made remarkable progress, however it is very difficult under the present conditions to completely remove leaks so that a leak detection test is now an indispensable step in the process of manufacturing a vacuum vessel. And also, after the completion of the apparatus, leaks often occur due to stress fatigue or corrosion of the material. Accordingly, it is necessary to carry out a leak detection test of large-sized and completely shaped apparatus in-situ under variously restricted conditions.
These leak detection tests must be able to specify a leak portion to several square centimeters of area or several centimeters of length, since the tests are premised on repair or exchange of leak location sections. This is comparatively simple in a small type of vacuum vessel, however, since, in the case of a large type of vacuum vessel, many hours and a great deal of labor are required, it is very important to improve this efficiency.
In the case of locating a very fine leak as below 10.sup.-15 Torr. 1/s (.about.10.sup.-5 cm.sup.3 (NTP)/s), a probe gas is usually employed. The probe gas method is classified roughly into an internal vacuum method and an interal pressure method. The former is to evacuate the interior of vacuum vessel to be tested by means of a vacuum pump and spray a probe gas, for example helium, partially onto the outside of the vessel to detect a leak spot, whereby the probe gas sprayed upon a leak portion flows into the interior of the vessel through the leak depending on the size of the leak and a detector, for example, a helium leak detector fitted on the vacuum vessel, acts thereupon. The latter, in contrast with the former, is to charge a probe gas, at a level of several atmospheric pressures, into the interior of the vacuum vessel to be tested and to inhale a probe gas flowing out from the vessel at the leak spot to the atmosphere with air through a probe nozzle (sniffer) and detect it. Each of the aforenoted methods has its merits and demerits. Particularly, in the case of applying the internal vacuum method to the leak detection test of large-sized apparatus, the equipment for vacuum evacuation, or the like are not only increased but also there is the problem of taking a lot of time required to start the test. On the other hand, the internal pressure (sniffer) method is simple and can be easily used independent of the size of the apparatus to be tested, but is comparatively lower in sensitivity so that the detection of very fine leaks, such as those below 10.sup.-7 Torr. 1/s (.about.10.sup.-7 cm.sup.3 (NTP)/s), is difficult.
As a result of researching the improvement of sensitivity in the internal pressure (sniffer) method in view of such point, the present inventors have found that the sensitivity can be elevated by about 10000 times than the conventional internal pressure (sniffer) method by using any one of helium, neon and hydrogen as a probe gas inserting a long capillary tube for keeping a flow rate constant, and using a vessel filled with a porous adsorbent which is cooled to a temperature below the liquid air temperature in series between a probe nozzle (sniffer) and a probe gas detector.